XY techniques for stereo audio recording as depicted in FIG. 9 use two coincident microphones 905a, 905b, that means, microphone capsules at the same place, i.e., two microphones 905a, 905b are mounted closely together to prevent almost totally time delay effects. At ideal conditions, this results in pure intensity stereo signals which can be summed to mono without comb-filter effects. This provides an unrestricted mono compatibility.
The XY technique uses two identical directional microphones 905a, 905b, e.g., cardioid or figure eight microphones. In relation to the recording axis 910, they are arranged at equal and opposed offset angles α with regard to their main directions 902a of stereo recording.
Or put differently, in relation to the recording axis 910 of the directional microphone arrangement formed by the pair of coincident directional microphones 905a, 905b, the two coincident microphones 905a, 905b are arranged such that the directional microphones' direct sound directions 902a are offset from the recording axis 910 by equal and opposed offset angles α. The recording axis 910 is also referred to as recording direction 910 of the microphone arrangement, central axis 910 for stereo recording or central direction 910 for stereo recording. Typical offset angles α are between −/+30° and −/+60° apart from the central direction of the stereo recording represented by the recording axis 910. For cardioid microphones, as shown in FIG. 9, the X microphone 905b pointing leftward supplies the left signal directly, and the Y microphone 905a pointing rightward supplies the right signal. The stereophonic properties depend on the directional characteristics of the microphones and the offset angle α. A reduced angle α will provide a minimum or narrow stereo image, whereas a large angle α will provide a wider stereo image. Thus, the angle β as depicted in FIG. 9, with β=2*α, represents a measure of the width of the stereo image, wherein the larger angle β, the larger the perceived width of the stereo image.
FIG. 10 shows the Blumlein stereo arrangement based on figure eight microphones. The L microphone 1005b has its main lobe pointing leftward and provides directly the left signal, while the R microphone 1005a offers the perfect symmetry and provides the right signal. The two microphones 1005a, 1005b correspond to the two directional microphones 905a, 905b of FIG. 9, and the direct sound directions 1002a of the two directional microphones 1005a, 1005b correspond to the direct sound directions 902a of the two directional microphones 905a, 905b of FIG. 9.
The Blumlein pair consists of an array of two matched microphones 1005a, 1005b of bi-directional pickup pattern, positioned 90° from each other, which corresponds to an offset angle α of 45° for each of the two directional microphones 1005a, 1005b with regard to the central direction 1010 for stereo recording. Ideally, the transducers should occupy the same physical space; as this is impossible the microphone capsules are placed as close to each other as physically possible, generally with one microphone 1005a centered directly above the other 1005b. The microphone array is oriented so that the line 1010 bisecting the angle β between the two microphones 1005a, 1005b, i.e. the recording axis 1010 points towards the sound source to be recorded. The pickup patterns of the microphone pair combined with their positioning, delivers a high degree of stereo separation in the source signal.
There are two typical microphone configurations in current mobile phones. The first one has only one single omnidirectional microphone which is used for communication and mono sound pickup. The other configuration is shown in FIG. 11. The mobile phone has two omnidirectional microphones, a main microphone 1113 and an auxiliary microphone 1107. The main microphone 1113 is arranged on the bottom side 1117 of the mobile phone which can be associated together with dock connectors 1109 for optional dock accessories and USB port 1111. The auxiliary microphone 1107 is located at the back side 1115 or back cover of the mobile phone together with camera 1105, e.g., 5 megapixel camera with autofocus, camera flash 1101 and speaker 1103. The main microphone 1113 is used for the sound pick up, i.e., for communication application as well as simple audio/video recording. The auxiliary microphone 1107 is used for noise cancellation and gain control.